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website/docs/language/expressions/conditionals.mdx

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+---
+page_title: Conditional Expressions - Configuration Language
+description: >-
+  Conditional expressions select one of two values. You can use them to define
+  defaults to replace invalid values.
+---
+
+# Conditional Expressions
+
+A _conditional expression_ uses the value of a boolean expression to select one of
+two values.
+
+> **Hands-on:** Try the [Create Dynamic Expressions](https://learn.hashicorp.com/tutorials/terraform/expressions?in=terraform/configuration-language&utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) tutorial on HashiCorp Learn.
+
+## Syntax
+
+The syntax of a conditional expression is as follows:
+
+```hcl
+condition ? true_val : false_val
+```
+
+If `condition` is `true` then the result is `true_val`. If `condition` is
+`false` then the result is `false_val`.
+
+A common use of conditional expressions is to define defaults to replace
+invalid values:
+
+```
+var.a != "" ? var.a : "default-a"
+```
+
+If `var.a` is an empty string then the result is `"default-a"`, but otherwise
+it is the actual value of `var.a`.
+
+## Conditions
+
+The condition can be any expression that resolves to a boolean value. This will
+usually be an expression that uses the equality, comparison, or logical
+operators.
+
+## Result Types
+
+The two result values may be of any type, but they must both
+be of the _same_ type so that Terraform can determine what type the whole
+conditional expression will return without knowing the condition value.
+
+If the two result expressions don't produce the same type then Terraform will
+attempt to find a type that they can both convert to, and make those
+conversions automatically if so.
+
+For example, the following expression is valid and will always return a string,
+because in Terraform all numbers can convert automatically to a string using
+decimal digits:
+
+```hcl
+var.example ? 12 : "hello"
+```
+
+Relying on this automatic conversion behavior can be confusing for those who
+are not familiar with Terraform's conversion rules though, so we recommend
+being explicit using type conversion functions in any situation where there may
+be some uncertainty about the expected result type.
+
+The following example is contrived because it would be easier to write the
+constant `"12"` instead of the type conversion in this case, but shows how to
+use [`tostring`](/language/functions/tostring) to explicitly convert a number to
+a string.
+
+```hcl
+var.example ? tostring(12) : "hello"
+```